A low power secondary battery in which one battery cell is made into a battery pack may be used as a power source for various portable electronic devices such as phones, laptop computers, and camcorders. A high power secondary battery in which several to tens of battery cells are made into a battery pack may be used as a power source for motor driven devices such as hybrid electric vehicles.
Depending on their external shape, secondary batteries may be classified into different types, for example, square, cylindrical, and pouch batteries. The secondary battery includes an electrode assembly in which positive and negative electrodes of a belt shape are wound with a separator interposed therebetween. The electrode assembly is placed inside a container, and a cap assembly having an external terminal is mounted on the container.
The positive and negative electrodes have coated regions of collectors which are coated with an active material, and uncoated regions which are not coated with an active material. Conductive taps are attached to the positive and negative uncoated regions, to collect electric current generated from the positive and negative electrodes.
For high power secondary batteries, the collecting structure has a plate-shaped collector plate instead of a conductive tap. The collector plate is electrically connected to the uncoated regions to reduce the internal resistance of the battery. However, because of this structure the separator may be broken during the welding process when the collector plate is fixed to the uncoated region.
In order to fix a collector plate to an uncoated region in an electrode assembly with a jellyroll configuration of a conventional secondary battery, the uncoated region at the end of the electrode is bent to provide surface contact with the collector plate, and then the contact portion is welded to the collector plate by laser welding. However, if the uncoated region is inappropriately bent, therefore causing insufficient surface contact with the collector plate, the separator may be exposed between the uncoated regions, and may be damaged due to contact with a high energy laser during laser welding.
The fracture of a separator can seriously damage the function of the secondary battery, especially a high power secondary battery for motor driven devices such as hybrid electric vehicles.